1. Field of the Invention
This invention pertains to suspension systems for tractors, trailers and other rubber tired vehicles and to the various novel elements which make up such a suspension system.
2. Description of the Prior Art
Most conventional suspension systems for mobile rubber tired vehicles have heretofore utilized steel springs or rubberized fabric "air bags." Such steel springs are in general constant rate type steel springs which are designed for the average load anticipated to be carried by the vehicle. For light lading, steel springs tend to be overly stiff causing accelerations to the lading which are damaging to it and heavily loaded vehicles have insufficient spring stiffness resulting in the vehicle frequently engaging the spring stops or bumpers causing impact forces to the vehicle and thus damaging the lading. Steel springs also have high vibration and shock transmissibility which causes equipment and lading damage. Air bag suspensions require ancillary shock absorbers and anti-bottom rubber stops. The air bags are costly, heavy, maintenance prone and require a source of compressed air.
Rubber sandwich type suspensions have been designed which to some extent alleviate the shortcoming of standard steel springs. An inherent disadvantage of these shear compression springs is their inherent lack of critical damping in the elastomer. These shear compression springs tend to be linear as applied and are nearly constant rate springs thus resulting in all of the inherent difficulties described above with reference to steel springs. Furthermore, shear compression springs require bonds between the steel plates and the elastomer members and in the event of failure of the bond, a failure could result in the suspension system. Almost all such systems require additional compression springs to provide full load stiffness.
In addition to the disadvantages regarding spring rate and load carrying capacity of prior art suspension systems, another important element in the consideration of a suspension system is the amount and type of damping in the system. There is much concern in the mobile vehicle industry regarding problems caused by dynamic forces which produce high frequency vibration, resonant motion, etc. Prior art damping systems to eliminate or reduce these problems have largely utilized constant force friction elements or costly hydraulic shock absorbers. Constant force damping has the disadvantage of being over-damped for light loads and under-damped for heavy loads. Hydraulic damping is velocity responsive rather than load responsive and also can allow damage to the lading for higher frequency forcing modes. Shear compression sandwich springs (whether chevron or flat) generally will provide only about 50% of the critical damping required for actual vehicle service without suffering from harmonic buildup.
Friction damping of dynamic forces action between a vehicle body and its rubber tired running gear have generally employed hydraulic shock absorbers as above or have been incorporated into the main load carrying spring of the vehicle as in U.S. Pat. Nos. 3,338,183; 3,517,620; 3,486,465 and 3,545,385.
Various types of elastomer springs have been designed heretofor. For example, elastomer springs are designed in the form of doughnuts, rectangles, etc. These configurations have not been highly successful for use in rubber tired vehicle suspensions. The doughnut, square and rectangular shapes occupy too great a volume for the tight space requirements of these suspensions and they all lack sufficient inherent damping. Square or rectangular shapes also provide less spring travel for a given height spring without over stressing them. Therefore, in limited space requirements, the desired deflection for a varying spring rate has not been practicably obtainable. Previously used elastomeric springs have seldom resulted in a satisfactory spring rate, even though they are non-linear, and have nearly always required ancillary shock absorbers.